Auto merge of #12303 - epage:config, r=joshtriplett

fix(script): Process config relative to script, not CWD

### What does this PR try to resolve?

This is part of the work for #12207.

When you put in your path `foo.rs`:
```rust
#!/usr/bin/env cargo

fn main() {}
```

You expect it to build once and then repeatedly run the same version.  However, `.cargo/config.toml` doesn't work like that (normally).  It is an environment file, like `.env`, and is based on your current working directory.  So if you run `foo.rs` from within a random project, it might rebuild due to RUSTFLAGS in `.cargo/config.toml`.

I had some concern about whether this current behavior is right or not and [noted this in the Pre-RFC](https://github.com/epage/cargo-script-mvs/blob/main/0000-cargo-script.md#unresolved-questions).  This came up again while we were [discussing editions on zulip](https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/cargo.20script.20and.20edition).  In looking further into this, it turns out we already have precedence for this with `cargo install --path <path>`.

### How should we test and review this PR?

The second commit has the fix, the docs, and a change to a test (from the first commit) to show that the fix actually changed behavior.

Author: bors

src/bin/cargo/commands/run.rs

@@ -93,13 +93,21 @@ pub fn is_manifest_command(arg: &str) -> bool {
         || path.file_name() == Some(OsStr::new("Cargo.toml"))
 }
 
-pub fn exec_manifest_command(config: &Config, cmd: &str, args: &[OsString]) -> CliResult {
+pub fn exec_manifest_command(config: &mut Config, cmd: &str, args: &[OsString]) -> CliResult {
     if !config.cli_unstable().script {
         return Err(anyhow::anyhow!("running `{cmd}` requires `-Zscript`").into());
     }
 
     let manifest_path = Path::new(cmd);
     let manifest_path = root_manifest(Some(manifest_path), config)?;
+
+    // Treat `cargo foo.rs` like `cargo install --path foo` and re-evaluate the config based on the
+    // location where the script resides, rather than the environment from where it's being run.
+    let parent_path = manifest_path
+        .parent()
+        .expect("a file should always have a parent");
+    config.reload_rooted_at(parent_path)?;
+
     let mut ws = Workspace::new(&manifest_path, config)?;
     if config.cli_unstable().avoid_dev_deps {
         ws.set_require_optional_deps(false);

src/doc/src/reference/unstable.md

@@ -1481,6 +1481,9 @@ A parameter is identified as a manifest-command if it has one of:
 - A `.rs` extension
 - The file name is `Cargo.toml`
 
+Differences between `cargo run --manifest-path <path>` and `cargo <path>`
+- `cargo <path>` runs with the config for `<path>` and not the current dir, more like `cargo install --path <path>`
+
 ### `[lints]`
 
 * Tracking Issue: [#12115](https://github.com/rust-lang/cargo/issues/12115)

tests/testsuite/script.rs

@@ -332,6 +332,47 @@ fn main() {
         .run();
 }
 
+#[cargo_test]
+fn use_script_config() {
+    let script = ECHO_SCRIPT;
+    let _ = cargo_test_support::project()
+        .at("script")
+        .file("script.rs", script)
+        .build();
+
+    let p = cargo_test_support::project()
+        .file(
+            ".cargo/config",
+            r#"
+[build]
+rustc = "non-existent-rustc"
+"#,
+        )
+        .file("script.rs", script)
+        .build();
+
+    // Verify the config is bad
+    p.cargo("-Zscript script.rs -NotAnArg")
+        .masquerade_as_nightly_cargo(&["script"])
+        .with_status(101)
+        .with_stderr_contains(
+            "\
+[ERROR] could not execute process `non-existent-rustc -vV` (never executed)
+",
+        )
+        .run();
+
+    // Verify that the config isn't used
+    p.cargo("-Zscript ../script/script.rs -NotAnArg")
+        .masquerade_as_nightly_cargo(&["script"])
+        .with_stdout(
+            r#"bin: [..]/debug/script[EXE]
+args: ["-NotAnArg"]
+"#,
+        )
+        .run();
+}
+
 #[cargo_test]
 fn test_line_numbering_preserved() {
     let script = r#"#!/usr/bin/env cargo